When Richard III died shortly after Robert's surrender, there were suspicions that Robert might have caused his brother's death.[6] Robert was accepted, however, as the sixth duke following his brother. But the civil war Robert I had started against his brother Richard III didn't stop when the two made peace.[6] There were still many feuds which had started between neighboring barons. These continued in Normandy during much of Robert’s reign.[6] It was also during this time that many of the lesser nobility left Normandy to seek their fortunes in southern Italy and elsewhere.[6] Soon after he became duke and possibly in revenge for supporting his brother, Robert I gathered an army and began raiding the lands of his uncle, Robert II, Archbishop of Rouen.[7] Only a brief truce allowed his uncle to leave Normandy in exile. The Archbishop then placed an edict of excommunication on Robert and all of Normandy.[7] The edict was only removed when Archbishop Robert was allowed to return and all his lands were restored.[7] Robert also attacked another powerful church leader, his cousin Hugo d'Ivry, Bishop of Bayeux. Robert banished him from Normandy.[8] Robert also seized a number of church properties belonging to the abbey of Fécamp.[9]

Robert's cousins, the Athelings Edward and Alfred, had been living in exile at the Norman Court. They were the sons of his aunt Emma of Normandy and Athelred, King of England.[13] At some point Robert attempted an invasion of England on their behalf but was stopped by unfavorable winds.[13] The king of England, Cnut the Great sent envoys to Duke Robert offering to settle half the Kingdom of England on Edward and Alfred.[14] After postponing the invasion he chose to also postpone the decision until after he returned from Jerusalem.[14]

Robert's attitude towards the Church changed completely since his uncle's return from exile.[15] To make up for what he had done to the Church earlier he restored all the property that he or his vassals had taken.[15] He also decided to make a pilgrimage to Jerusalem to make up for his sins.[16] After making his illegitimate son William his heir, he set out on his pilgrimage.[17] He travelled by way of Constantinople and reached Jerusalem. He fell seriously ill and died on the return journey at Nicaea on 2 July 1035.[b][17] His son William, aged about eight, succeeded him.[19]

According to the historian William of Malmesbury, decades later, Robert's son William sent a mission to Constantinople and Nicaea to return his father's body to Normandy for burial.[20]Permission was granted, but, having travelled as far as Apulia (Italy) on the return journey, the envoys learned that William himself had died.[20] They then decided to reinter Robert's body in Italy.[20]

↑In some sources he is called 'Robert the Devil' (French: le Diable). This is wrong. Robert I was never known by the nickname 'the devil'. 'Robert the Devil' was a fictitious person who was later mixed up with with Robert I, Duke of Normandy sometime near the end of the Middle Ages.[1]